Analysis of water, fat, protein and salt contents in processed cheese or natural cheese has conventionally been carried out according to the methods prescribed for particular chemical compositions (i.e., official methods of analysis) comprising, for example, the steps of chopping respective samples and treating them with chemicals. However, these methods of the prior art have been inconvenient from various viewpoints. For example, intricate procedures and a relatively long time are usually required to obtain the desired analysis results.
Recently, a method utilizing near infrared analysis for principal constituent analysis has been developed and such method has been employed in the cheese industry to determine values of chemical components other than the salts. However, many of these attempts have used a wavelength range of 1100 to 2500 nm, relying on reflectivity as the means of measurement (Joseph F. Frank J. Dairy Sci. 65.1110-1116, 1982) and with this method there is no means for a determination of the content of salts in cheese.
Such conventional reflectivity measurement utilizes a long wavelength region of the near infrared and has been inevitably accompanied, in the case of milk, with standard deviations of 0.076% for the entire solid components, 0.048% for fat, 0.046% for protein and 0.053% for lactose and, in the case of natural cheese, 1.53% for water, 1.51% for fat and 2.20% for protein. Thus, the precision of analysis has been significantly low in the case of cheese.
In this conventional method, intricate operation of sampling is required, since a specular surface, as smooth as possible, must be provided in order the reflectivity method of measurement may be effectively carried out, and no improvement of such well known method has been achieved to overcome the low precision of analysis of cheese components and to accelerate the operation of the measurement.
For milk, on the other hand, the so-called milkoscan analysis has been practically employed as a relatively speedy method of measurement, but, when this method is employed to determine the fat content in milk, fat globules must be crushed under a uniform pressure of approximately 300 kg/cm.sup.2 for homogenization and special equipment is necessary to achieve this.
Furthermore, the ash content of milk can not be determined by using such infrared method and, consequently, the analyzers commonly used in practice have been unable to handle such components except as fixed values.
Moreover, when the near infrared region is used in a range of 1100 to 2500 nm, as commonly been used, for analysis of milk components, the optical path length for the sample has had to be less than 0.5 mm because of the wavelength used and this requirement has often caused sample error. As a consequence, it has been difficult for the subsequent run of an analysis to provide high reproducibility because of the small quantity of sample within that optical path length of a previous run.
In this manner, both the infrared and the near infrared region from 1100 to 2500 nm which have been commonly utilized have encountered the above-mentioned problems. On the other hand, a determination of milk components utilizing the near infrared region having a wavelength from 700 to 1200 nm has not been reported in the art.
The object of the present invention is to determine values of principal components of skim milk, milk, cream and cheese without any pretreatment such as homogenization or treatment with chemicals.
As will be apparent from the foregoing, in the conventional determination utilizing the near infrared region from 1100 to 2500 nm or the infrared region, the method of measurement has been limited to the reflectivity method, and depending on the particular subjects, when a transmittance method of measurement can be employed, the optical path for a sample has had to be set to a length less than 0.5 mm. Therefore, it has usually been difficult to achieve a sample path length sufficient to obtain high reproducibility. In addition, dairy products cover a variety of items such as milk and skim milk in liquid state, on one hand, and butter, cheese etc. in solid state, on the other hand. Particularly for solid state items, the optical path for the sample must be prolonged and, to this end, the wavelength of near infrared must be correspondingly shortened. However, the near infrared region from 1800 to 2500 nm spans a range corresponding to the first absorption band which is most sharp in the NIR region and includes a strong absorption band substantially over the entire range while the near infrared region from 600 to 1100 nm corresponds to the second overtone bands in which the absorption is too weak to obtain adequate information. Accordingly, the reflectivity method of measurement has not been employed in the latter region.
In view of these problems, the inventors have devoted themselves to improvements and developed a novel method utilizing the near infrared having a wavelength from 700 to 1200 nm to determine a quantity of radiation transmitted by a sample and then using results of this determination to make a calibration curve by multiple linear regression so that the above-mentioned drawback of the second overtone bands may be adequately compensated and even the solid dairy products such as cheese can be nondestructively analyzed.